1. Field of the Invention
The present invention relates generally to projection exposure apparatus for use in the process of manufacturing semiconductor devices and, more particularly, to a projection exposure apparatus provided with a projection optical system suitable for use in transferring micro patterns.
2. Related Background Art
Reducing projection exposure apparatus have conventionally been used in the manufacturing of semiconductor devices composed of micro patterns, such as large scale integrated circuits (LSI's) and very large scale integrated circuits (VLSI's). To handle micro patterns which will be far finer in the future, great efforts have been directed to the improvement of the accuracy and stability with which micro patterns are transferred. For example, a great number of efforts have been made at reducing the wavelength of exposing illumination light or increasing the numerical apertures (NA's) of projection optical systems. To achieve stable transference of such a micro pattern, it is also necessary to use a projection optical system having high resolution and a projecting image having high contrast. For this reason efforts have been made at finding optimum exposure conditions by variously investigating the state of illumination. For example, Japanese Utility Model Laid-Open No. 61-151 discloses a method of determined illumination conditions. In the disclosed method, a so-called o value corresponding to the ratio of the NA of the projection optical system to the NA of an illumination optical system is selected to adjust the NA's of both optical systems, thereby providing adequate balance of resolution and contrast with respect to a predetermined pattern.
The aforesaid conventional apparatus allows the illumination optical system to be optimized to some extent but, as the NA of the projection optical system increases, depth of focus becomes shallower. As a result, when the micro pattern formed on a reticle is to be transferred onto a wafer by exposure, extremely strict focusing conditions are needed. This leads to the problem that, if even a slight amount of focus variation occurs, the micro pattern will not be transferred accurately.
In general, patterns to be transferred are not formed of simple periodic structures only and, in a major part of patterns, a plurality of micro patterns having different periodic structures coexist. In addition, general projection optical systems tend to vary their best focus positions, depending on the linewidths of individual micro patterns. Accordingly, the best focus positions differ with the finenesses of patterns to be transferred, with the result that extremely strict adjustment and control steps are indispensable to the manufacturing processes.
It is, therefore, always necessary to detect focus extremely strictly and to correct a focus position highly accurately. This leads to an increase in the complexity and size of the apparatus, an increase in the duration of process time and a deterioration in throughput.
An invention intended to cope with the above-described problems is disclosed in a commonly assigned U.S. Pat. application Ser. No. 07/451,166 filed Dec. 15, 1989 and now U.S. Pat. No. 4,965,630. In this invention, the amount of spherical aberration of a projection optical system is made excessive, whereby the resolution of an image formed on a resist having a thickness can be improved in excess of the limit of resolution which appears on the resist as a result of multiple reflection therewithin in the absence of any aberration.
However, it has been found that the prior invention still has the disadvantage that resolution deteriorates substantially if extremely fine patterns are transferred.